Inspired by the “semi” thread, I want to ask something I’ve been wondering lately.
In the past few years, I’ve been seeing trucks with smaller “extra” wheels, mounted just in front of the main load-carrying wheels, and not even touching the ground. Like this:
http://www.bandag.com/download/ImagesLogos/dumptruck.jpg
What are those for?
The lift axle may be lowered to provide extra support and meet weight-per-axle requirements when heavily loaded. At other times, they can be raised for improved gas mileage, or to save wear and tear on the unneeded wheels.
The main reason these wheels are retractable is that the vehicle cannot make tight manouvers with these wheels extended. Due to their positioning they slide sideways instead of rolling.
Without them many of the trucks wouldn’t be allowed on the county roads during certain times of the year. Adding the extra axle allows them on roads they couldn’t travel otherwise. The bit about why they are retractable has been covered.
Damn. I always just assumed those were spare tires.
Thanks, all. I’ve never noticed any that had the extra axle in the lowered position - I wasn’t aware it could do that.
I thought they mounted them just to look badass. Like people put spoilers and spinners on their cars.
Around here, they’re called tag axles, and are used along with tarps. Heading into the quarry, both are up. Heading out, axle down and tarp over, or a large fine is waiting for you, if an officer is 1/4 mile down the road.
Note-many truckers hauling commodities other than stone employ tag axles and tarps. The above was meant to be illustrative.
A tangential question: is GAWR (Gross Axle Weight Rating) the same as ESAL (Equivalent Single Axle Load)?
So GAWR is a rating for the vehicle, and ESAL is a design estimate for the road. Got it.
But if it was in the lowered position, how would you know that it could be raised? It would just look like another axle.
If you watch, you’ll usually see them lower them as they get on a freeway (or soon after) and they tend to raise them as they are heading down the off ramp.
A fair number of long-distance motor coaches (eg: Greyhound and Trailways) have tag axles as well. One nifty thing that the bus builders are doing is designing the suspension so the tag is automatically lifted when the transmission is in reverse or if the forward speed is under 15 MPH to improve maneuverability.
Due to state laws you will see LOTS of these in Michigan. Don’t know the details, but it was something that really stood out when I had to travel there on buisiness.